Thumbnails not working in Home page

Hi, I have some problems in retrieving the pages thumbnails in my home page. Everything works properly when thumbanils are displayed in other pages, but related thumbanails disappears when current page is home. How can I solve this problem? I’m actually working on a wp_menu_nav, where there are mixed pages titles with their secondary-image thumbnails.

This is the code put in function.php file:

<?php

// nav menu walker
class My_Walker extends Walker_Nav_Menu
{
    function start_el(&$output, $item, $depth, $args) {
        global $wp_query;
        $indent = ( $depth ) ? str_repeat( "\t", $depth ) : '';

        $class_names = $value = '';

        $classes = empty( $item->classes ) ? array() : (array) $item->classes;

        $class_names = join( ' ', apply_filters( 'nav_menu_css_class', array_filter( $classes ), $item ) );
        $class_names = ' class="' . esc_attr( $class_names ) . '"';

        $output .= $indent . '<li id="menu-item-'. $item->ID . '"' . $value . $class_names .'>';

        $attributes  = ! empty( $item->attr_title ) ? ' title="'  . esc_attr( $item->attr_title ) .'"' : '';
        $attributes .= ! empty( $item->target )     ? ' target="' . esc_attr( $item->target     ) .'"' : '';
        $attributes .= ! empty( $item->xfn )        ? ' rel="'    . esc_attr( $item->xfn        ) .'"' : '';
        $attributes .= ! empty( $item->url )        ? ' href="'   . esc_attr( $item->url        ) .'"' : '';
        $attributes .= ' data-id="'. esc_attr( $item->object_id        ) .'"';
        $attributes .= ' data-slug="'. esc_attr(  basename(get_permalink($item->object_id )) ) .'"';

		$post_id = $item->object_id;
		ob_start();
		echo '<br><img src="' . MultiPostThumbnails::get_post_thumbnail_url(get_post_type(), 'secondary-image', $post_id) . '" />';
		$myStr = ob_get_contents();
		ob_end_clean();

        $item_output = $args->before;
        $item_output .= '<a'. $attributes . '>';
        $item_output .= $args->link_before . apply_filters( 'the_title', $item->title, $item->ID ) . $myStr . $args->link_after;
        $item_output .= '</a>'; /* This is where I changed things. */
        $item_output .= $args->after;

        $output .= apply_filters( 'walker_nav_menu_start_el', $item_output, $item, $depth, $args );
    }}

?>

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